In the process of production of a steel sheet, in the hot-rolling process, the final rolled steel sheet was cooled to a predetermined temperature by a cooling system provided between the finish rolling mill and the coiler and then was coiled up by the coiler.
In the hot-rolling process of a steel sheet, the mode of cooling by this cooling system (for example, providing an air cooling zone for holding the sheet at an intermediate holding temperature in the middle of cooling, making the cooling stop temperature, the coiling temperature, etc.) is becoming an important factor in deciding the mechanical characteristics of steel sheet.
Further, in the case of a cold-rolled steel sheet, in the annealing process performed after the cold-rolling, the mode of cooling at the cooling system after holding at the heating furnace (cooling rate and cooling stop temperature) is also becoming an important factor in deciding the mechanical characteristics of steel sheet.
This cooling is controlled by operating water valves or gas valves of the cooling system to spray the surface of the steel sheet with water or a gas. In this case, the basic heat transfer equation based on the coefficient of heat transfer and specific heat is used and the sheet thickness, sheet width, pass rate, entry-side temperature, cooling stop target temperature, and other input data are processed to determine the number of valves to operate.
However, it is very difficult to precisely control the temperature pattern or the end-of-cooling temperature corresponding to the changes in input conditions for each coil and within a coil.
As a control method to improve the temperature precision, Japanese Patent Publication (A) No. 7-214132 reports a method of ON/OFF control of valves when the predicted temperature is deviated from. Further, Japanese Patent Publication (A) No. 59-7414 reports the technology of installing a measurement system of the temperature and amount of transformation during cooling and revising the cooling amount based on the actual value.
On the other hand, as technology which aims at improvement of the precision of the predicted temperature, Japanese Patent Publication (A) No. 9-267113 reports a control method which estimates the coefficient of heat transfer based on the actual values of the finishing temperature, intermediate temperature, coiling temperature, and the like, while Japanese Patent Publication (A) No. 2000-317513 reports a control method which estimates the coefficient of heat transfer in water cooling in a transition state to nucleate boiling and film boiling.
However, these all relate to the coefficient of heat transfer.
Further, on the other hand, the estimates of the amount of heat generated from materials are not sufficiently advanced. This also becomes a cause of a decrease of the precision of temperature prediction.
Japanese Patent Publication (A) No. 4-274812 reports a method which predicts the amount of transformation heat using a transformation fraction found from a transformation fraction measuring device attached to the cooling system, while Japanese Patent Publication (A) No. 8-103809 reports, similarly for a method of obtaining a grasp of the transformation heat, a method which uses a prediction model of the transformation process to predict the transformation fraction by computation and estimate the transformation heat.
However, with these methods, it is not possible to consider the dependency of the specific heat on the transformation fraction and it is not possible to accurately estimate the heat from a steel sheet.
In regard to this, in Nippon Steel Tech. Rep., No. 67, (1995), 49 (M. Suehiro et al.) and ISIJ Int., Vol. 32, No. 3, (1992), 433 (M. Suehiro et al.), in order to estimate the temperature dependency of specific heat in addition to the transformation heat, the specific heat of the ferrite phase divided into the specific heat of magnetic transformation and the specific heat without magnetic transformation and the effect of the transformation fraction on the specific heat of magnetic transformation is introduced.
However, this idea is predicated on dividing the specific heat by the transformation fraction, so the specific heat of the austenite phase is not considered. The precision of temperature prediction at the beginning of transformation and at the high temperature range ends up becoming lower and estimation of the specific heat of magnetic transformation is very difficult.